In recent years, apparatuses that use a secondary battery are increasing in number. This is due to the fact that the communication apparatus represented by a laptop personal computer are becoming more and more smaller, lighter and therefore portable. A secondary battery is charged in the following manner. A charge remaining in the secondary battery is detected and a control is carried out for sequentially charging based on this remaining charge.
FIG. 7 is a block diagram showing a structure of a switch driver in a conventional charge control system. As shown in FIG. 7, a collector of a pnp transistor TR1 is connected to an adapter 1 via a node Pa while an emitter thereof is connected to a plus terminal of a secondary battery 2 via the node Pb. Furthermore, a base of the pnp transistor TR1 is connected to a collector of the pnp transistor TR2 via the node Pc while an emitter of an npn transistor TR2 is grounded via a node Pe. A control section 3 is connected to a base of the npn transistor TR2 via a node Pd.
The control section 3 supplies a current to the base of the npn transistor TR2 to turn ON the npn transistor TR2 with the result that a current flows from the collector of the npn transistor TR2 to the emitter thereof. When the current flows with the turning ON of the npn transistor TR2, a base current of the pnp transistor TR1 increases, and the pnp transistor TR1 is turned ON with the result that a charging current iab flows to the secondary battery 2 from the adapter 1 and the secondary battery 2 is charged.
On the other hand, unless the control section 3 supplies a current to the base of the npn transistor TR2, the npn transistor TR2 is maintained in an OFF state with the result that the current does not flow from the collector of the npn transistor TR2 to the emitter thereof. As a consequence, the base current of the pnp transistor TR1 does not flow, and the pnp transistor TR1 is maintained in the OFF state with the result that the charging current iab does not flow from the adapter 1 to the secondary battery 2. The ON and OFF state of the pnp transistor TR1 is controlled with the ON and OFF control of the npn transistor TR2 by the control section 3, with the result that the charging of the secondary battery 2 by the charging current iab can be controlled with the ON and OFF thereof.
However, in the conventional switch driver described above, since the potential of the node Pc falls all of a sudden to a level that is nearly the grounding level because of the turning ON of the npn transistor TR2, the base current of the pnp transistor TR1 also flows all of a sudden. As a consequence, the charge current iab which flows through the pnp transistor TR1 abruptly increases and the charging current iab is overshot due to a transition phenomenon and the pnp transistor also vibrates largely.
Generally, in a charge control system that uses such a switch driver, a terminal voltage and a charging current iab of the secondary battery 2 are detected in order to protect the secondary battery 2. When over-voltage or over-current is detected, then an operation for dealing with an unusual state, namely suspension of charging, is carried out. However, when the overshooting described above is generated then there is a problem that the charge control system erroneously detects that an over-current has generated with the result that a normal charging operation is hindered.